HIV infection is characterized by high rates of viral turnover throughout the disease process, eventually leading to CD4 depletion and disease progression. Wei X, Ghosh S K, Taylor M E, et al. (1995) Nature 343, 117–122 and Ho D D, Naumann A U, Perelson A S, et al. (1995) Nature 373, 123–126. The aim of antiretroviral therapy is to achieve substantial and prolonged suppression of viral replication. Achieving sustained viral control is likely to involve the use of sequential therapies, generally each therapy comprising combinations of three or more antiretroviral drugs. Choice of initial and subsequent therapy should, therefore, be made on a rational basis, with knowledge of resistance and cross-resistance patterns being vital to guiding those decisions. The primary rationale of combination therapy relates to synergistic or additive activity to achieve greater inhibition of viral replication. The tolerability of drug regimens will remain critical, however, as therapy will need to be maintained over many years.
In an untreated patient, some 1010 new viral particles are produced per day. Coupled with the failure of HIV reverse transcriptase (RT) to correct transcription errors by exonucleolytic proofreading, this high level of viral turnover results in 104 to 105 mutations per day at each position in the HIV genome. The result is the rapid establishment of extensive genotypic variation. While some template positions or base pair substitutions may be more error prone (Mansky L M, Temin H M (1995) J Virol 69, 5087–5094) (Schinazi R F, Lloyd R M, Ramanathan C S, et al. (1994) Antimicrob Agents Chemother 38, 268–274), mathematical modeling suggests that, at every possible single point, mutation may occur up to 10,000 times per day in infected individuals.
For antiretroviral drug resistance to occur, the target enzyme must be modified while preserving its function in the presence of the inhibitor. Point mutations leading to an amino acid substitution may result in changes in shape, size or charge of the active site, substrate binding site or in positions surrounding the active site of the enzyme. Mutants resistant to antiretroviral agents have been detected at low levels before the initiation of therapy. (Mohri H, Singh M K, Ching W T W, et al. (1993) Proc Natl Acad Sci USA 90, 25–29) (Nájera I, Richman D D, Olivares I, et al. (1994) AIDS Res Hum Retroviruses 10, 1479–1488) (Nájera I, Holguin A, Quiñones-Mateu E, et al. (1995) J Virol 69, 23–31). However, these mutant strains represent only a small proportion of the total viral load and may have a replication or competitive disadvantage compared with wild-type virus. (Coffin J M (1995) Science 267, 483–489). The selective pressure of antiretroviral therapy provides these drug-resistant mutants with a competitive advantage and thus they come to represent the dominant quasi species (Frost S D W, McLean A R (1994) AIDS 8, 323–332) (Kellam P, Boucher C A B, Tijnagal J M G H (1994) J Gen Virol 75, 341–351) ultimately leading to a rebound in viral load in the patient.
Early development of antiretroviral therapy focused on inhibitors of reverse transcriptase. Both nucleoside and non-nucleoside inhibitors of this enzyme showed significant antiviral activity (DeClerq, E. (1992) AIDS Res. Hum. Retrovir. 8:119–134). However, the clinical benefit of these drugs had been limited due to drug resistance, limited potency, and host cellular factors (Richman, D. D. (1993) Ann. Rev. Pharm. Tox. 32:149–164). Thus inhibitors targeted against a second essential enzyme of HIV were urgently needed.
In 1988, the protease enzyme of HIV was crystallized and its three-dimensional structure was determined, (Navia M A, Fitzgerald P M D, McKeever B M, Leu C T, Heimbach J C, Herber W K, Sigal I S, Darke P L, Springer J P (1989) Nature 337:615–620 and Winters M A, Schapiro J M, Lawrence J, Merigan T C (1997) In Abstracts of the International Workshop on HIV Drug Resistance, Treatment Strategies and Eradication, St. Petersburg, Fla.) allowing for the rapid development of protease inhibitors. Initially, it was hypothesized that HIV protease, unlike reverse transcriptase, would be unable to accommodate mutations leading to drug resistance. This is not the case, and to date over 20 amino acid substitutions in the HIV protease have been observed during treatment with the currently available protease inhibitors. The genetic pattern of mutations conferring resistance to these protease inhibitors is complex, and cross-resistance between structurally different compounds occurs.
Protease Inhibitors
HIV protease was classified as an aspartic proteinase on the basis of putative active-site homology (Toh H, Ono M, Saigo K, Miyata T (1985) Nature 315:691), its inhibition by peptastin (Richards A D, Roberts R, Dunn B M, Graves M C, Kay J (1989) FEBS Lett 247:113), and its crystal structure (Navia M A, Fitzgerald P M D, McKeever B M, Lau C T, Heimbach J C, Herber W K, Sigal I S, Darke P L, Springer J P (1989) Nature 337:615–620). The enzyme functions as a homodimer composed of two identical 99-amino acid chains (Debouck C, Navia M A, Fitzgerald P M D, McKeever B M, Leu C T, Heimbach J C, Herber W K, Sigal I S, Darke P L, Springer J P (1988) Proc. Natl. Acad. Sci. USA 84:8903–8906), with each chain containing the characteristic Asp-Thr-Gly active-site sequence at positions 25 to 27 (Toh H, Ono M, Saigo K, Miyata T (1985) Nature 315:691).
HIV protease processes gag (p55) and gag-pol (p160) polyprotein products into functional core proteins and viral enzymes (Kohl N E, Diehl R E, Rands E, Davis L J, Hanobik M G, Wolanski B, Dixon R A (1991) J. Virol. 65:3007–3014 and Kramer R A, Schaber M D, Skalka A M, Ganguly K, Wong-Staal F, Reddy E P (1986) Science 231:1580–1584). During or immediately after budding, the polyproteins are cleaved by the enzyme at nine different cleavage sites to yield the structural proteins (p17, p24, p7, and p6) as well as the viral enzymes reverse transcriptase, integrase, and protease (Pettit S C, Michael S F, Swanstrom R (1993) Drug Discov. Des. 1:69–83).
An asparagine replacement for aspartic acid at active-site residue 25 results in the production of noninfectious viral particles with immature, defective cores (Huff J R (1991) AIDS J. Med. Chem. 34:2305–2314, Kaplan A H, Zack J A, Knigge M, Paul D A, Kempf D J, Norbeck D W, Swanstrom R (1993) J. Virol. 67:4050–4055, Kohl N E, Emini E A, Schleif W A, Davis L J, Heimbach J C, Dixon R A, Scolnik E M, Sigal I S (1988) Proc. Natl. Acad. Sci. USA 85:4686–4690, Peng C, Ho B K, Chang T W, Chang N T (1989) J. Virol. 63:2550–2556). Similarly, wild-type virus particles produced by infected cells treated with protease inhibitors contain unprocessed precursors and are noninfectious (Crawford S, Goff S P (1985) J. Virol. 53:899–907, Gottlinger H G, Sodroski J G, Haseltine W A (1989) Proc. Natl. Acad. Sci. USA 86:5781–5785, Katoh I Y, Yoshinaka Y, Rein A, Shibuya M, Odaka T, Oroszlan S (1985) Virology 145:280–292, Kohl N E, Emini E A, Schleif W A, Davis L J, Heimbach J C, Dixon R A, Scolnik E M, Sigal I S (1988) Proc. Natl. Acad. Sci. USA 85:4686–4690, Peng C, Ho B K, Chang T W, Chang N T (1989) J. Virol. 63:2550–2556, Stewart L, Schatz G, Wogt V M (1990) J. Virol. 64:5076–5092). Unlike reverse transcriptase inhibitors, protease inhibitors block the production of infectious virus from chronically infected cells (Lambert D M, Petteway, Jr. S R, McDanal C E, Hart T K, Leary J J, Dreyer G B, Meek T D, Bugelski P J, Bolognesi D P, Metcalf B W, Matthews T J (1992) Antibicrob. Agents Chemother. 36:982–988). Although the viral protease is a symmetric dimer, it binds its natural substrates or inhibitors asymmetrically (Dreyer, G B, Boehm J C, Chenera B, DesJarlais R L, Hassell A M, Meek T D, Tomaszek T A J, Lewis M (1993) Biochemistry 32:937–947, Miller M J, Schneider J, Sathyanarayana B K, Toth M V, Marshall G R, Clawson L, Selk L, Kent S B, Wlodawer A (1989) Science 246:1149–1152). These findings together with the knowledge that amide bonds of proline residues are not susceptible to cleavage by mammalian endopeptidases gave rise to the first class of HIV-1 protease inhibitors based on the transition state mimetic concept, with the phenylalanine-proline cleavage site being the critical nonscissile bond (Roberts N A, Martin J A, Kinchington D, Broadhurst A V, Craig J C, Duncan I B, Galpin S A, Handa B K, Kay J, Krohn A, Lambert R W, Merett J H, Mills J S, Parkes K E B, Redshaw S, Ritchie A J, Taylor D L, Thomas G J, Machin P J (1990) Science 248:358–361).
Amino Acids Implicated in Resistance to Protease Inhibitors.
As new protease inhibitors are developed, the ability of certain amino acid substitutions to confer resistance to the inhibitor is usually determined by several methods, including selection of resistant strains in vitro, site-directed mutagenesis, and determination of amino acid changes that are selected during early phase clinical trials in infected patients. While some amino acid substitutions are specifically correlated with resistance to certain protease inhibitors (see below), there is considerable overlap between sets of mutations implicated in resistance to all approved protease inhibitors. Many investigators have attempted to classify these mutations as either being “primary” or “secondary”, with varying definitions. For example, some investigators classify as primary mutations which are predicted, based on X-ray crystallographic data, to be in the enzyme active site with the potential for direct contact with the inhibitor. (e.g. D3ON, G48V, I50V, V82A/F/S/T, I84V, N88S, L90M). Secondary mutations are usually considered as being compensatory for defects in enzyme activity imposed by primary mutations, or as having enhancing effects on the magnitude of resistance imparted by the primary mutations (e.g. L10I/F/R/V, K20I/M/R/T, L24I, V32I, L33F/V, M36I/L/V, M46I/L/V, I47V, I54L/V, L63X, A71T/V, G73A/S/T, V77I, N88D). Lists of mutations and corresponding inhibitors are maintained by several organizations, for example: Schinazi et al., Mutations in retroviral genes associated with drug resistance, Intl. Antiviral News 1999,7:46–69 and Shafer et al., Human Immunodeficiency Virus Reverse Transcriptase and Protease Sequence Database, Nucleic Acids Research 1999, 27(1), 348–352.
Saquinavir
Saquinavir, developed by Hoffmann-La Roche, was the first protease inhibitor to undergo clinical evaluation, demonstrating that HIV-1 protease was a valid target for the treatment of HIV infection (Jacobsen H, Brun-Vezinet F, Duncan I, Hanggi M, Ott M, Vella S, Weber J, Mous J (1994) J. Virol. 68:2016–2020). Saquinavir is a highly active peptidomimetic protease inhibitor with a 90% inhibitory concentration (IC90) of 6 nM (id). In vitro, saquinavir can select for variants with one or both of two amino acid substitutions in the HIV-1 protease gene, a valine-for-glycine substitution at position 48 (G48V), a methionine-for-leucine substitution at residue 90 (L90M), and the double substitution G48V-L90M (Eberle J, Bechowsky B, Rose D, Hauser U, vonder Helm K, Guertler L, Nitschko H (1995) AIDS Res. Hum. Retroviruses 11:671–676, Jacobsen H, Yasargil K, Winslow D L, Craig J C, Kroehn A, Duncan I B, Mous J (1995) Virology 206:527–534, Turriziani O, Antonelli G, Jacobsen H, Mous J, Riva E, Pistello M, Dianzani F (1994) Acta Virol. 38:297–298). In most cases, G48V is the first mutation to appear, and continued selection results in highly resistant double-mutant variants. A substitution at either residue results in a 3- to 10-fold decreased susceptibility to the inhibitor, whereas the simultaneous occurrence of both substitutions causes a more severe loss of susceptibility of >100-fold (id).
In vivo, saquinavir therapy appears to select almost exclusively for mutations at codons 90 and 48 (id, Jacobsen H, Hangi M, Ott M, Duncan I B, Owen S, Andreoni M, Vella S, Mous J (1996) J. Infect. Dis. 173:1379–1387, Vella S, Galluzzo C, Giannini G, Pirillo M F, Duncan I, Jacobsen H, Andreoni M, Sarmati L, Ercoli L (1996) Antiviral Res. 29:91–93). Saquinavir-resistant variants emerge in approximately 45% of patients after 1 year of monotherapy with 1,800 mg daily (Craig I C, Duncan I B, Roberts N A, Whittaker L (1993) In Abstracts of the 9th International Conference on AIDS, Berlin, Germany, Duncan I B, Jacobsen H, Owen S, Roberts N A (1996) In Abstracts of the 3rd Conference of Retroviruses and Opportunistic Infections, Washington, D. D., id, Mous J, Brun-Vezinet F, Duncan I B, Haenggi M, Jacobsen H, Vella S (1994) In Abstracts of the 10th International Conference on AIDS, Yokohama, Japan). The frequency of resistance is lower (22%) in patients receiving combination therapy with zidovudine, zalcitabine, and saquinavir (Collier A C, Coombs R, Schoenfeld D A, Bassett R L, Joseph Timpone M S, Baruch A, Jones M, Facey K, Whitacre C, McAuliffe V J, Friedman H M, Merigan T C, Reichmann R C, Hooper C, Corey L (1996) N. Engl. J. Med. 334:1011–1017). In contrast to in vitro-selected virus, where the G48V mutation is the first step to resistance, the L90M exchange is the predominant mutation selected in vivo while the G48V (2%) or the double mutant (<2%) is rarely found (id). In another recent study of in vivo resistance during saquinavir monotherapy no patient was found to harbor a G48V mutant virus (Ives K J, Jacobsen H, Galpin S A, Garaev M M, Dorrell L, Mous J, Bragman K, Weber J N (1997 J. Antimicrob. Chemother. 39:771–779). Interestingly, Winters et al. (id) observed a higher frequency of the G48V mutation in patients receiving higher saquinavir doses as monotherapy. All patients (six of six) who initially developed G48V also acquired a V82A mutation either during saquinavir treatment or after switching to either indinavir or nelfinavir. An identical mutational pattern was found in another study during saquinavir monotherapy (Eastman P S, Duncan I B, Gee C, Race E (1997) In Abstracts of the International Workshop on HIV Drug Resistance, Treatment Strategies and Eradication, St. Petersburg, Fla.). Some residues represent sites of natural polymorphism of the HIV-1 protease (positions 10, 36, 63, and 71) and appear to be correlated to the L90M mutation (id). Another substitution, G73S, has been recently identified and may play a role in saquinavir resistance in vivo. Isolates from five patients with early saquinavir resistance and those from two patients with induced saquinavir resistance after a switch of therapy to indinavir carried the G73S and the L90M substitutions Dulioust A, Paulous S, Guillemot L, Boue F, Galanaud P, Clavel F (1997) In Abstracts of the International Workshop on HIV Drug Resistance, Treatment Strategies and Eradication, St. Petersburg, Fla.).
Ritonavir
Ritonavir, developed by Abbott Laboratories, was the second HIV protease inhibitor to be licensed in the United States. Ritonavir is a potent and selective inhibitor of HIV protease that is derived from a C2-symmetric, peptidomimetic inhibitor (Ho D D, Toyoshima T, Mo H, Kempf D J, Norbeck D, Chen C M, Wideburg N E, Burt S K, Erickson J W, Singh M K (1994) J. Virol. 68:2016–2020). In vitro activity has been demonstrated against a variety of laboratory strains and clinical isolates of HIV-1 with IC90s of 70 to 200 nM (Kuroda M J, El-Farrash M A, Cloudhury S, Harada S (1995) Virology 210:212–216.
Resistant virus generated by serial, in vitro passages is associated with specific mutations at positions 84, 82, 71, 63, and 46 (Markowitz M, Mo H, Kempf D J, Norbeck D W, Bhat T N, Erickson J W, Ho D D (1995) J. Virol. 69:701–706). The I84V substitution appeared to be the major determinant of resistance, resulting in a 10-fold reduction in sensitivity to ritonavir. Addition of the V82F mutation confers an even greater level of resistance, up to 20-fold. The substitutions M46I, L63P, and A71V, when introduced into the protease coding region of wild-type NL4–3, did not result in significant changes in drug susceptibility. Based on replication kinetics experiments, these changes are likely to be compensatory for active-site mutations, restoring the impaired replicative capacity of the combined V82F and I84V mutations.
Indinavir
Indinavir, developed by Merck & Co., is the third HIV protease inhibitor licensed in the United States. Indinavir is a potent and selective inhibitor of HIV-1 and HIV-2 proteases with Ki values of 0.34 and 3.3 nM, respectively (Vacca J p, Dorsey B D, Schleif W A, Levin R B, McDaniel S L, Darke P L, Zugay J, Quintero J C, Blahy O M, Roth E, Sardana V V, Schlabach A J, Graham P I, Condra J H, Gotlib L, Holloway M K, Lin J, Chen L-w, Vastag K, Ostobich D, Anderson PS, Emini E A, Huff J R (1994) Proc. Natl. Acad. Sci. USA 91:4096–4100). The drug acts as peptidomimetic transition state analogue and belongs to the class of protease inhibitors known as HAPA (hydroxyaminopentane amide) compounds (ibid). Indinavir provides enhanced aqueous solubility and oral bioavailability and in cell culture exhibits an IC95 of 50 to 100 nM (Emini E A, Schleif W A, Deutsch P, Condra J H (1996) Antiviral Chemother. 4:327–331.
Despite early reports of a lack of in vitro resistance by selection with indinavir (id), Tisdale et al. (Tisdale M, Myers R E, Maschera B, Parry N R, Oliver N M, Blair E D (1995) Antibicrob. Agents Chemother. 39:1704–1710) were able to obtain resistant variants during selection in MT-4 cells with substitutions at residues 32, 46, 71, and 82. At least four mutations were required to produce a significant loss of susceptibility (6.1-fold compared with the wild type). The mutation at position 71, described as compensatory (Markowitz M, Mo H, Kempf D J, Norbeck D W, Bhat T N, Erickson J W, Ho D D (1995) J. Virol. (id), appeared to contribute phenotypic resistance and also to improve virus growth. Emini et al. (id) and Condra et al. (Condra J H, Holder D J, Schleif W A, Blahy O M, Danovich R M, Gabryelski L J, Graham D J, Laird D, Quintero J C, Rhodes A, Robbins H L, Roth E, Shivaprakash M, Yang T, Chodakewitz J A, Deutsch P J, Leavitt R Y, Massari F e, Mellors J W, Squires K E, Steigbigel R T, Teppler H, Emini E A (1995) Nature 374:569–571) found by constructing mutant HIV-1 clones that at least three mutations at residues 46, 63, and 82 were required for the phenotypic manifestation of resistance with a fourfold loss of susceptibility.
Nelfinavir
Nelfinavir, developed by Agouron Pharmaceuticals, is a selective, nonpeptidic HIV-1 protease inhibitor that was designed by protein structure-based techniques using iterative protein crystallographic analysis (Appelt K R, Bacquet J, Bartlett C, Booth C L J, Freer S T, Fuhry M M, Gehring M R, Herrmann S M, Howland E F, Janson C A, Jones T R, Kan C C, Kathardekar V, Lewis K K, Marzoni G P, Mathews D A, Mohr C, Moomaw E W, Morse C A, Oatley S J, Ogden R C, Reddy M R, Reich S H, Schoettlin W S, Smith W W, Varney M D, Villafranca J E, Ward R W, Webber S, Webber S E, Welsh K M, White J (1991) J. Med. Chem. 34:1925–1928). In vitro, nelfinavir was found to be a potent inhibitor of HIV-1 protease with a Ki of 2.0 nM (Kaldor S W, Kalish V J, Davies J F, Shetty B V, Fritz J E, Appelt K, Burgess J A, Campanale K M, Chirgadze N Y, Clawson D K, Dressman B A, Hatch S D, Khalil D A, Kosa M B, Lubbehusen P P, Muesing M A, Patrick A K, Reich S H, Su K S, Tatlock J H (1997) J. Med. Chem. 40:3979–3985). The drug demonstrated antiviral activity against several laboratory and clinical HIV-1 and HIV-2 strains with 50% effective concentrations ranging from 9 to 60 nM (Patick A K, Boritzki T J, Bloom L A (1997) Antimicrob. Agents Chemother. 41:2159–2164). Nelfinavir exhibits additive-to-synergistic effects when combined with other antiretroviral drugs (Partaledis J A, Yamaguchi A K, Tisdale M, Blair E E, Falcione C, Maschera B, Myers R E, Pazhanisamy S, Futer O, Bullinan A B, Stuver C M, Byrn R A, Livingston D J (1995) J. Virol. 69:5228–5235). Preclinical data showed high levels of the drug in mesenteric lymph nodes and the spleen and good oral bioavailability (Shetty B V, Kosa M B, Khalil D A, Webber S (1996) Antimicrob. Agents Chemother. 40:110–114).
In vitro, following 22 serial passages of HIV-1NL4-3 in the presence of nelfinavir, a variant (P22) with a sevenfold reduced susceptibility was isolated. After an additional six passages a variant (P28) with a 30-fold-decreased susceptibility to nelfinavir was identified (Patick A K, Ho H, Markowitz M, Appelt K, Wu B, Musick L, Kaldor S, Reich S, Ho D, Webber S (1996) Antimicrob. Agents Chemother. 40:292–297). Sequence analysis of the protease gene from these variants identified in decreasing frequency the substitutions D30N, A71V, and I84V for the P22 variant and mutations M46I, I84V/A, L63P, and A71V for the P28 variant. Antiviral susceptibility testing of recombinant mutant HIV-1NL4-3 containing various mutations resulted in a fivefold-increased 90% effective concentration for the I84V and D30N single mutants and the M46I/I84V double mutant, whereas no change in susceptibility was observed with M46I, L63P, or A71V alone (ibid).
Amprenavir
Amprenavir is a novel protease inhibitor developed by Vertex Laboratories and designed from knowledge of the HIV-1 protease crystal structure (Kim E E, Baker C T, Dyer M D, Murcko M A, Rao B G, Tung R D, Navia M A (1995) J. Am. Chem. Soc. 117:1181–1182). The drug belongs to the class of sulfonamide protease inhibitors and has been shown to be a potent inhibitor of HIV-1 and HIV-2, with IC50s of 80 and 340 nM, respectively. The mean IC50 for amprenavir against clinical viral isolates was 12 nM (St. Clair M H, Millard J, Rooney J, Tisdale M, Parry N, Sadler B M, Blum M R, Painter G (1996) Antiviral Res. 29:53–56) HIV-1 variants 100-fold resistant to amprenavir have been selected by in vitro passage experiments (id). DNA sequence analysis of the protease of these variants revealed a sequential accumulation of point mutations resulting in amino acid substitutions L10F, M46I, I47V, and I50V. The key resistance mutation in the HIV-1 protease substrate binding site is I50V. As a single mutation it confers a two- to threefold decrease in susceptibility (ibid). The other substitutions did not result in reduced susceptibility when introduced as single mutations into an HIV-1 infectious clone (HXB2). However, a triple protease mutant clone containing the mutations M46I, I47V, and I50V was 20-fold less susceptible to amprenavir than wild-type virus. The I50V mutation has not been frequently reported in resistance studies with other HIV protease inhibitors. Kinetic characterization of these substitutions demonstrated an 80-fold reduction in the inhibition constant (Ki) for the I50V single-mutant protease and a 270-fold-reduced Ki for the triple mutant M46I/I47V/I50V, compared to the wild-type enzyme (Pazhanisamy S, St6uvr C M, Cullinan A B, Margolin N, Rao B G (1996) J. Biol. Chem. 271:17979–17985). The single mutants L10F, M46I, and I47V did not display reduced affinity for amprenavir. The catalytic efficiency (kcat/Km) of the I50V mutant was decreased up to 25-fold, while the triple mutant M46I/I47V/I50V had a 2-fold-higher processing efficiency than the I50V single mutant, confirming the compensatory role of the M46I-and-I47V mutation. The reduced catalytic efficiency (kcat/Km) for these mutants in processing peptides appeared to be due to both increased Km and decreased kcat values.
Viral Fitness
The relative ability of a given virus or virus mutant to replicate is termed viral fitness. Fitness is dependent on both viral and host factors, including the genetic composition of the virus, the host immune response, and selective pressures such as the presence of anti-viral compounds. Many drug-resistant variants of HIV-1 are less fit than the wild-type, i.e. they grow more slowly in the absence of drug selection. However, since the replication of the wild-type virus is inhibited in the presence of drug, the resistant mutant can outgrow it. The reduction in fitness may be a result of several factors including: decreased ability of the mutated enzyme (i.e. PR or RT) to recognize its natural substrates, decreased stability of the mutant protein, or decreased kinetics of enzymatic catalysis. See Back et al., EMBO J. 15: 4040–4049, 1996; Goudsmit et al., J. Virol. 70: 5662–5664, 2996; Maschera et al., J. Biol. Chem. 271: 33231–33235, 1996; Croteau et al., J. Virol. 71: 1089–1096, 1997; Zennou et al., J. Virol. 72: 300–3306, 1998; Harrigan et al., J. Virol. 72: 3773–3778, 1998; Kosalaraksa et al., J. Virol. 73: 5356–5363, 1999; Gerondelis et al., J. Virol. 73: 5803–5813, 1999. Drug resistant viruses that are less fit than wild type may be less virulent i.e. they may cause damage to the host immune system more slowly than a wild type virus. Immunological decline may be delayed after the emergence of drug resistant mutants, compared to the rate of immunological decline in an untreated patient. The defect causing reductions in fitness may be partially or completely compensated for by the selection of viruses with additional amino acid substitutions in the same protein that bears the drug resistance mutations (for example, see Martinez-Picado et al., J. Virol. 73:3744–3752, 1999), or in other proteins which interact with the mutated enzyme. Thus, amino acids surrounding the protease cleavage site in the gag protein may be altered so that the site is better recognized by a drug-resistant protease enzyme (Doyon et al., J. Virol. 70: 3763–3769, 1996; Zhang et al., J. Virol. 71: 6662–6670, 1997; Mammano et al., J, Virol. 72: 7632–7637, 1998).
Integrase
Integration of viral DNA into the host chromosome is a necessary process in the HIV replication cycle (Brown, P. O., 1997, in Retroviruses; Coffin, J. M., Hughes, S. H. & Varmus, H. E., eds., Cold Spring Harbor Lab. Press, Plainview, N.Y., 161–203). The key steps of DNA integration are carried out by the viral integrase protein, which, along with protease and reverse transcriptase, is one of three enzymes encoded by HIV. Combination antiviral therapy with protease and reverse transcriptase inhibitors has demonstrated the potential therapeutic efficacy of antiviral therapy for treatment for AIDS (Vandamme, A. M., Van Vaerenbergh, K. & De Clerq, E., 1998, Antiviral Chem. Chemother. 9, 187–203). However, the ability of HIV to rapidly evolve drug resistance, together with toxicity problems, requires the development of additional classes of antiviral drugs. Integrase is an attractive target for antivirals because it is essential for HIV replication and, unlike protease and reverse transcriptase, there are no known counterparts in the host cell. Furthermore, integrase uses a single active site to accommodate two different configurations of DNA substrates, which may constrain the ability of HIV to develop drug resistance to integrase inhibitors. However, unlike protease and reverse transcriptase, for which several classes of inhibitors have been developed and cocrystal structures have been determined, progress with the development of integrase inhibitors has been slow. A major obstacle has been the absence of good lead compounds that can serve as the starting point for structure-based inhibitor development. Although numerous compounds have been reported to inhibit integrase activity in vitro, most of these compounds exhibit little specificity for integrase and are not useful as lead compounds (Pommier, Y., Pilon, A. A., Bajaj K, K., Mazumder, A. & Neamati, N., 1997, Antiviral Chem. Chemother 8).
HIV-1 integrase is a 32-kDa enzyme that carries out DNA integration in a two-step reaction (Brown, P. O., ibid.). In the first step, called 3′ processing, two nucleotides are removed from each 3′ end of the viral DNA made by reverse transcription. In the next step, called DNA strand transfer, a pair of transesterification reactions integrates the ends of the viral DNA into the host genome. Integrase is comprised of three structurally and functionally distinct domains, and all three domains are required for each step of the integration reaction (Engelman, A. Bushman, F. D. & Craigie, R., 1993, EMBO J. 12, 3269–3275). The isolated domains form homodimers in solution, and the three-dimensional structures of all three separate dimers have been determined (Dyda, F., Hickman, A. B. Jenkins, T. M., Engelman, A., Craigie, R. & Davies, D. R., 1994, Science 226, 1981–1986; Goldgur, Y. Dyda, Hickman, A. B., Jenkins, T. M., Craigie, R. & Davies, D. R., 1998, Proc. Natl. Acad. Sci., USA 95, 9150–9154; Maignan, S., Guilloteau, J. P., Zhou-Liu, Q., Clement-Mella, C. & Mikol, V., 1998, J. Mol. Biol. 282, 259–368; Lodi, P. J., Ernst, J. A., Kuszewski, J., Hickman, A. B., Engelman, A., Craigie, R., Clore, G. M. & Gronenborn, A. M. 1995 Biochemistry 34, 9826–9833; Eijkelenboom, A. P., Lutzke, R. A., Boelens, R., Plasterk, R. H., Kaptein, R. & Hard, K. 1995 Nat. Struct. Biol. 2, 807–810; Cai, M. L., Zheng, R., Caffrey, M., Craigie, R., Clore, G. M. & Gronenborn, A. M., 1997 Nat. Struct. Biol. 4, 839–840). Although little is known concerning the organization of these domains in the active complex with DNA substrates, integrase is likely to function as at least a tetramer (Dyda, F., Hickman, A. B. Jenkins, T. M., Engelman, A., Craigie, R. & Davies, D. R., 1994, Science 226, 1981–1986). Extensive mutagenesis studies mapped the catalytic site to the core domain (residues 50–212), which contains the catalytic residues D64, D116, and E152 (Engelman, A. & Craigie R., 1992, J. Virol. 66, 6361–6369; Kulkosky, J., Jones, K. S., Katz, R. A., Mack, J. P. & Skalka, A. M., 1992, Mol. Cell Biol 12, 2331–2338). The structure of this domain of HIV-1 integrase has been determined in several crystal forms (Dyda, F., Hickman, A. B. Jenkins, T. M., Engelman, A., Craigie, R. & Davies, D. R., 1994, Science 226, 1981–1986; Goldgur, Y. Dyda, Hickman, A. B., Jenkins, T. M., Craigie, R. & Davies, D. R., 1998, Proc. Natl. Acad. Sci., USA 95, 9150–9154; Maignan, S., Guilloteau, J. P., Zhou-Liu, Q., Clement-Mella, C. & Mikol, V., 1998, J. Mol. Biol. 282, 259–368).
Hazuda et al. (Science 287: 646–650, 2000) have described compounds (termed L-731, 988 and L-708,906) which specifically inhibit the strand-transfer activity of HIV-1 integrase and HIV-1 replication in vitro. Viruses grown in the presence of these inhibitors display reduced inhibitor susceptibility and bear mutations in the integrase coding region at amino acid positions 66 (T66I), 153 (S153Y), and 154 (M154I). Site-directed mutants of a laboratory strain of HIV-1 (HXB2) with these amino acid changes confirmed their direct role in conferring reduced integrase inhibitor susceptibility. In addition some of these mutants displayed delayed growth kinetics, suggesting that viral fitness was impaired.
It is an object of this invention to provide a drug susceptibility and resistance test capable of showing whether a viral population in a patient is either more or less susceptible to a given prescribed drug. Another object of this invention is to provide a test that will enable the physician to substitute one or more drugs in a therapeutic regimen for viruses that show altered susceptibility to a given drug or drugs after a course of therapy. Yet another object of this invention is to provide a test that will enable selection of an effective drug regimen for the treatment of HIV infections and/or AIDS. Yet another object of this invention is to provide the means for identifying alterations in the drug susceptibility profile of a patient's virus, in particular identifying changes in susceptibility to protease inhibitors. Still another object of this invention is to provide a test and methods for evaluating the biological effectiveness of candidate drug compounds which act on specific viruses, viral genes and/or viral proteins particularly with respect to alterations in viral drug susceptibility associated with protease inhibitors. It is also an object of this invention to provide the means and compositions for evaluating HIV antiretroviral drug resistance and susceptibility.
It is an object of this invention to provide a method for measuring replication fitness which can be adapted to viruses, including, but not limited to human immunodeficiency virus (HIV), hepadnaviruses (human hepatitis B virus), flaviviruses (human hepatitis C virus) and herpesviruses (human cytomegalovirus). This and other objects of this invention will be apparent from the specification as a whole.